An oil depot in the port area of the Nigerian city of Lagos has been hit by a major explosion and fire.The blast, which shook buildings in the Apapa area, happened during a transfer of fuel, the National Emergency Management Agency (Nema) said.Firefighters have been at the scene and four people were wounded in the blast, AFP news agency reports.Many tankers dock at jetties in the port in Lagos to load and unload petroleum products.Willem Auret, who witnessed the blast from a ship on its way to Snake Island in Apapa, said he saw a tanker barge catch fire at about 11:00 local time (10:00 GMT)."The fire started slowly and then expanded into chaos, exploding more than once," he told the BBC."After the initial explosion, which I caught on camera, there was a secondary explosion," he said.It took about an hour and a half for the port authorities to arrive on the scene, he said."First one tugboat arrived... then several others joined it in an attempt to extinguish the fire. They seem to have the fire under control now."Nema's Akande Iyiola told the BBC that the oil depot at Tin Can Island port where the explosion occurred was owned by the petroleum company MRS Oil."I felt the explosion from where we are," Charles Osagie, who works at an import-and-export office in the district, told AFP.Nigeria is one of the world's biggest oil producers, but imports refined petrol.